This thesis examines the way in which Germanic dialects, which were brought to Britain by settlers in the post-Roman period and which ultimately evolved into Old English, came to predominate in the southern and eastern parts of the island over a stretch of roughly two centuries spanning the fifth to the seventh. It uses theories of language contact and creolization to explain why Celtic languages left very little influence on Old English, despite the fact that Celtic speakers came into contact with the Germanic speakers and were incorporated into their communities. Evidence is provided by linguistics (including toponymy), history, archaeology, and recent breakthroughs in archaeogenetic research. The latter in particular has firmly established that mass migration from the North Sea coast areas of the European continent to Britain was the primary reason for cultural change. To support the argument, paper also examines analogous scenarios of language contact with better historical attestation.